While PVC is generally produced by suspension polymerization method, emulsion polymerization method, or bulk polymerization method, particularly a suspension polymerization method and emulsion polymerization method have widely been used from the advantages that the reaction heat can readily be removed and products in which the amount of impurities is small can be obtained.
Suspension polymerization method and emulsion polymerization method are usually conducted by charging VCM together with an aqueous medium, dispersing agent or emulsifier, and polymerization initiator into a polymerization vessel provided with a stirrer, and polymerizing the VCM while stirring and maintaining it at a predetermined temperature.
Usually, polymerization reaction is not continued to the point where VCM is converted by 100% to PVC, and it is terminated at a stage where production efficiency is high, that is, terminated at a polymerization conversion of 80 to 95%. After termination of the polymerization reaction, residual monomers in a polymerization vessel are separated from a PVC slurry (mixed dispersion comprising mainly PVC particles and an aqueous medium) and then recovered. However, the PVC slurry generally contains a few % of unreacted residual monomers.
Subsequently, the aqueous medium in the PVC slurry is mechanically separated, and the residue is dried by a hot-air drying or one of various other methods to form PVC powders. On this occasion, VCM is contained in the separated aqueous medium described above, exhaust air in a hot-air drying, and the PVC powders each in an extent which is made into a matter of concern on the grounds of environmental sanitation, or clearly considered to be harmful.
Various methods have been proposed to completely remove the discharges formed in such production and VCM in the PVC powders, or to decrease the content of VCM down to the extent at which it is harmless to environmental sanitation.
As the method for removing and recovering unreacted residual monomers more efficiently, methods for removing and recovering residual monomers from a PVC slurry by using a treating tower which has plural trays made of perforated plates therein and has a steam ejecting port at its bottom portion were proposed (Laid-open Japanese Patent Publication No. Sho 54-8693 and Laid-open Japanese Patent Publication No. Sho 56-22305).
Characteristics of these methods are trays made of perforated plates in which the base are constructed by perforated plates and partition walls are installed on the perforated plates so that treating passages are formed in a zigzag; a PVC slurry is exposed to steam ejected from a lower portion through perforations of the perforated plates, during the time when the PVC slurry flows along the treatment passages on the trays made of perforated plates, and residual monomers contained in the PVC slurry are evaporated and separated.
Further, a method and an apparatus for removing residual monomers were proposed in which bubbling caused in the upper section of a tower is suppressed by making its diameter larger than that of the lower section of the tower, thereby stabilized operation and prevention of incorporation of degradated particles formed by the bubbling become possible (Laid-open Japanese Patent Publication No. Hei 07-224109).
Contact time of the slurry with steam necessary for removing VCM in a PVC slurry differs according to the grade of PVC. Generally, a slurry of a PVC having a low polymerization degree is hardly demonomerized and a slurry of a PVC having a high polymerization degree is readily demonomerized.
However, in the methods described above, it is impossible to efficiently treat different grade of PVC slurries having different contact times with steam necessary for removing residual monomers in the same apparatus because residence time of PVC slurries is maintained constant by the treating passages and partition walls on the trays made of perforated plates.
That is, when a PVC slurry from which residual monomers are difficultly removed is treated in an apparatus which is designed for treating a PVC slurry from which residual monomers can readily be removed, contact time with steam is insufficient, and unreacted residual monomers can not sufficiently be removed from the PVC slurry. Conversely, when a PVC slurry from which residual monomers can readily be removed is treated in an apparatus which is designed for treating a PVC slurry from which residual monomers are difficultly removed, PVC particles contact with steam for a time longer than necessary after the residual monomers were removed, and thermal degradation of the PVC is caused, and quality of PVC products deteriorates.
In the plants for manufacturing PVC, usually many times a plural grades are produced by the same facility. Accordingly, if PVC powders remain in the equipment when a product grade is changed to a different one, PVC particles of different grades are incorporated to cause an inconvenience such as fish eyes, thereby to depreciate the value of products.
Even in the case where the same grade is treated, if PVC particles adhere within a tower for removing residual monomers and remain for long period of time, the particles cause thermal degradation by steam, and the particles unwillingly discolor brown (hereinafter, discoloration of particles by thermal degradation is referred to as coloration). If colored PVC particles adhered within the tower are fell off the wall surface of the tower and mixed in a PVC slurry, the value of products is reduced at the time of their processing.
In addition, in the method of demonomerization described above in which bubbling is suppressed, a PVC slurry disperses when it is introduced in a tower from a PVC slurry introducing portion, adheres on the internal wall of the tower, and causes such a problem of colored PVC particles as described above, whereas the residence degradation of a part of PVC can be prevented.